Corrugated paperboard is widely used as a material for fabricating containers and for other packaging applications. Corrugated paperboard is strong, lightweight, relatively inexpensive, and may be recycled. Conventional corrugated paperboard is constructed of two opposing liners and an intervening fluted sheet secured together using an adhesive. The adhesive is typically a starch-based adhesive applied as a liquid. Accordingly, heat is transferred to the paperboard to dry or set the adhesive during the manufacturing of the paperboard.
A conventional so-called double-facer for setting the adhesive includes a series of steam heating chests over which the paperboard is advanced. A conveyor belt engages the upper surface of the board and advances the board along the heating chests. A series of rolls is typically used to provide backing pressure to the back side of the conveyor belt. Accordingly, the paperboard is pressed into contact with the underlying steam heating chests.
Unfortunately, the steam heating chests have a tendency to bow or deflect due to temperature differences thereby producing low quality paperboard. This problem is explained in greater detail in U.S. Pat. No. 5,456,783 to Sissons. The Sissons patent discloses a significant advance in the art of corrugated paperboard manufacturing wherein a series of contact assemblies provide backing pressure to the conveyor belt rather than conventional backing rolls. The contact assemblies include independently mounted and biased contact shoes, mounted in side-by-side relation. The contact shoes can readily conform to any bowing of the steam heating chests. The contact assemblies are readily installed, and operated with greatly reduced maintenance, especially compared to conventional backing rolls and their associated bearings. Because heat transfer to the paperboard is also increased, less heating chests may be used and ambient energy losses reduced further.
U.S. Pat. No. 5,256,240 to Shortt discloses a plurality of fluid filled bladders for applying the backing pressure to a conveyor belt of a double-facer. The Shortt patent discloses that in certain applications the conveyor belt may be omitted; however, the patent fails to disclose how to advance the corrugated paperboard sheet along its path of travel against the heating chests without a conveyor belt.
Those familiar with corrugated paperboard manufacturing appreciate that the conveyor belt may absorb a significant amount of heat and moisture in operation. Accordingly, the conveyor belt contributes to energy losses. Moreover, the conveyor belt may have a relatively short life and may be relatively expensive to periodically replace. In addition, as the belt wears, the quality of the paperboard may be reduced, such as when using conventional backing rolls, for example. The drawbacks associated with a conventional conveyor belt have simply been endured for lack of a more advantageous alternative.